The membrane photocurrent of the outer segments of the rod photoreceptors of Bufo marinus will be measured with the suction microelectrode technique of Baylor, Lamb and Yau. The impulse response (linear response) to light impulses will be measured under experimental conditions that allow comparison with the kinetics of activation and deactivation of rod disk membrane phosphodiesterase, measured under comparable conditions in companion experiments. The hypothesis will thus be tested that the PDE enzyme system is part of the mechanism of visual transduction.